The invention relates to cancellation of interference from neighboring cells in a wireless communications system.
It is well known that the performance of cellular systems is limited due to the presence of intercell interference. For example, in the downlink, users at the edge of the cell may experience interference from neighboring cells at received power levels similar to that of the signal from the serving cell. This is especially the case for densely planned systems employing low frequency-reuse factors. Indeed, in the limit, a frequency reuse of one may be used. While a reuse of one is desirable to maximize the amount of time/frequency resources available to each cell in the system, the intercell interference problem it creates naturally results in a lowering of the data rates achievable for users at the edges of the cell. Transmissions to users with low signal to noise plus interference ratios (SNIR) require information redundancy and hence a low code rate to achieve the desired decoding quality (measured for example as block error rate, BLER), resulting in a corresponding reduction in the data rate.
The use of multiple-input multiple-output (MIMO) antenna systems is also well known. In these systems, data may be transmitted to a user over sets of channels existing between nTx transmit antennas and nRx receive antennas. There are thus a total of nTx x nRx channels which make up the composite MIMO channel set. Multiple simultaneous data streams may be transmitted over the channel set if the channels of the set are sufficiently statistically independent and uncorrelated.
It is further well known that the ability of the system to successfully transmit multiple simultaneous and different data streams over the MIMO channel set is also a function of the channel SNIR. Successful transmission of parallel streams is more likely in channels with high SNIR and less likely in channels with low SNIR. Thus, the gain of MIMO transmission (in terms of achievable link throughput), compared to its transmit and receive diversity counterparts, is increased for cases of higher SNIR and higher channel decorrelation. For low SNIR or high channel correlation, the gains of MIMO diminish and instead transmission of a single data stream can result in better overall performance than transmission of multiple parallel data streams, and thus is preferred. Note that the multiple channels in the set may still be used to provide transmit/receive diversity benefits; however, no attempt is made to transmit multiple parallel data streams in this case.
FIGS. 1A and 1B illustrate one example of two-stream MIMO transmission over a 2×2 channel set, and one example of single-stream non-MIMO transmission over the same channel set, respectively. The primary difference is that in the two-stream MIMO case, each transmit antenna is conveying different information, whereas in the single stream case, the information transmitted by each antenna is the same (although the actual signal waveforms may differ).
It is further known that systems may dynamically switch between single and multi-(e.g. dual) stream transmission according to variations observed in the channel SNIR or as the statistical correlation between the channels in the channel set is seen to vary. In this way, users experiencing poorer radio conditions (low SNIR and/or high channel correlation) will receive single-stream transmissions and users with good radio conditions (high SNIR and/or low channel correlation) will be able to exploit multi-stream transmission to achieve higher data rates and link throughput.